Such systems have previously been described and have been used for injection quantity measurements in internal combustion engines.
DE-AS 1 798 080 thus describes an electronically controlled through-flow meter having an inlet and an outlet between which a rotational displacement device configured as a gear pump is arranged, and a piston arranged in a measuring chamber in a line extending parallel to the displacement device. The displacement of the piston in the measuring chamber is measured by an optical sensor to determine the through-flow rate. On the basis of this signal, the rotational speed of the gear pump is continuously readjusted via an evaluation and control unit so that the piston is, if possible, always returned to its initial position so that only small flows are produced in the circuitous line. The through-flow within a predetermined time interval is calculated on the basis of the number of rotations or partial rotations of the gear pump measured by an encoder as well as the known volume flow of the gear pump during one rotation.
A through-flow meter of such a design is also described in DE 103 31 228 B3. For determining the exact injection discharge rates, the gear pump is adjusted to a constant rotational speed before the injection process is started so that the movement of the piston is subsequently measured and used to determine the injection discharge rates. A pressure sensor and a temperature sensor are additionally arranged in the measuring chamber, the measuring values of which are also supplied to the computing unit to calculate and correct the injection discharge rates.
It has, however, turned out that determining of the injection discharge rates is still subject to minor errors which are attributable to variations of the viscosity or the density of the measured fluid that have thus far not been considered.